Nanocrystalline cellulose (NCC), or cellulose nanocrystals (CNC), are extracted as a colloidal suspension by (typically sulfuric) acid hydrolysis of lignocellulosic materials, such as bacteria, cotton, or wood pulp. NCC is comprised of cellulose, a linear polymer of β(1→4) linked D-glucose units, whose chains are arranged to form crystalline and amorphous domains. Colloidal suspensions of cellulose crystallites form a chiral nematic structure upon reaching a critical concentration. Hydrogen bonding between the cellulose chains can stabilize the local structure in NCC, and plays a key role in the formation of crystalline domains. The iridescence of NCC self-assemblies is typically characterized by the finger-print patterns, where the patch work of bright and dark regions is typical of spherulitic behavior of fibrillar crystals in which the molecules are packed with their axes perpendicular to the fibrillar axis. NCC is also characterized by high crystallinity (between 85 and 97%, typically greater than 90%) approaching the theoretical limit of the cellulose chains (Hamad and Hu, Can. J. Chem Engrg., 2010, 88: 392-402).
Owing to its unique strength and self-assembly properties, NCC can act as high-performance reinforcement in polymer systems. The major obstacles to NCC application in composite manufacture are: (1) aggregation of NCC particles, (2) poor dispersion of the hydrophilic NCC particles in mostly hydrophobic polymer matrices, and (3) poor interfacial adhesion between NCC and polymer. Different approaches have been followed to increase NCC's dispersion and interaction with polymer matrices. Use of surfactants is a simple enough method, but a large amount of surfactant is normally required which would negatively impact the strength of the resulting composite. Surface modification, on the other hand, generally involves reaction with the hydroxyl groups on the NCC surface. Silanes have, for example, been employed to graft hydrophobic groups onto the NCC surface. Moreover, polymers with hydroxyl reactive groups have been used as well, such as polyethylene glycol (PEG) (see Araki, J. et al. Langmuir, 2001, 17 (1), 21-27, Polycaprolactone (PCL) (see Habibi, Y. et al. Biomacromolecules, 2008, 9 (7), 1974-1980) and poly(propylene) (PP) (see Ljungberg, N. et al. Biomacromolecules 2005, 6 (5), 2732-2739). Such modifications can make NCC more hydrophobic and give NCC reasonable stability in organic solvents. However, these reactions (i) generally involve several, intricate steps, (ii) are therefore costly, and (iii) have limited scalability.
The International Standards Organisation (ISO) has stipulated that the use of the term cellulose nanocrystals (CNC), should replace nanocrystalline cellulose (NCC), however the two are used herein interchangeably.